There will be a before and after Kepler Era in astronomy. Today, with the release of 1,202 exoplanet candidates from data collected with the Kepler spacecraft over 140 days of observation, we have just entered in a new age of astronomy.

The Kepler spacecraft is the 10th NASA Discovery mission launched in March 2009 which was designed to search for exoplanets by measuring almost continuously the brightness of 156,453 stars in a small 12 degree diameter patch of the sky. The 0.95m-telescope is able to detect attenuation of the host star located in the Cygnus, Lyra, Draco constellations which could be due to the transit of an exoplanet passing between its star and us. A transit gives direct estimate on the size of the exoplanet and its orbit.

On February 1, 2011, the Kepler team released the data from the first 136 days of operation of the spacecraft. An article (Borucki and 64 colleagues, 2011) summarizing the key results from these observations after a detailed data processing and careful analysis is about to be submitted. In this post, I am summarizing the key results of this remarkable work.

The Field of View of Kepler with the location of the
exoplanet candidates and their family classification per size
(credit: NASA-Kepler team) (Click on image for larger view)

The dim attenuation of light of the host start due to the passage of an exoplanet was identified by the Kepler team for 1,202 potential exoplanets. Because for most of them, the radial velocity measurement (due to the wobbling of the star due to the exoplanets) and the close vicinity of the star are not yet known, the genuineness of these exoplanets are not yet fully confirmed. A careful statistical analysis allowed the team to conclude that 80-90% of these candidates are probably real. You may know that the Extrasolar Planets Encyclopedia contains currently 519 exoplanets which were discovered over 15 years of observations. In 4 months of operation, the Kepler mission has tripled the number of known exoplanets.

More than just discoveries, the Kepler data gives us the opportunity to study statistically the properties of these exoplanets. For the first time, astronomers are able to define families of exoplanets arbitrary classified per size which are:

The figure below summarizes the candidate size versus orbital period and candidate equilibrium temperature. The horizontal lines mark the limit between these families of exoplanets. (Click on image for larger view)

Orbital period and estimated temperature of the exoplanet candidates

Several exoplanets orbit around M- and K- type stars, which have lower temperatures than our Sun. Consequently, the surface temperature of the exoplanets could permit the presence of liquid water. In their paper, the Kepler team lists ~60 candidates with sizes ranging from Earth-size to larger than that of Jupiter which are in the Habitability Zone of their host star. This is obviously one of the most remarkable results of this survey.

There is much more to say about this catalog of exoplanet candidates. For instance, we can now say that stars in our Milky Way galaxy are more likely to host small exoplanets since 75% of the Kepler catalog exoplanets are smaller than Neptune, with a peak of exoplanets only 2-3 times larger than Earth.

Using model predictions which take into account the probability of having the correct geometry to detect these exoplanets, the Kepler team extrapolated that 6% of the stars in our Milky way have Earth- and super-Earth size exoplanets, 17% of them have Neptune-size candidates and only 4% of them have Jupiter-size exoplanets.

However, only 14% of the host stars have multi-candidate systems, including one multi-planet system named Kepler-11 with 6 planets, which was also announced today. The lack of more distant exoplanets in the survey is probably an observational bias since Kepler was in operation for only 4 months, so the longest orbital period attainable currently with the Kepler data is ~140 day, corresponding to a distance of 0.5 AU from an host star. Since this is less than the distance between our sun and Venus, Kepler-11 is a surprising system with 6 Neptune-size exoplanets (Rp~2-4 Re) orbiting in a very compact configuration around their Sun-like host star. Multi-planet systems seem to be all astonishingly unique and interesting.

The Kepler mission is still in operation and new data is collected with a cadence of 30 min, on a time baseline significantly larger than this first catalog. We should expect in a few years to have a more accurate and complete catalog, which might contain smaller exoplanets, exoplanets orbiting at larger distances from their host star and even satellites of these exoplanets. The involvement of a large community of astronomers is now needed to confirm and refine this catalog by collecting additional observations.

Tonight if you look at the sky, you may consider to pause from your hectic life and think that among these 2,500 stars that can be seen without a telescope, a third of them may host an exoplanet. If it is not enough to appreciate the plurality of other worlds, you should remember that our Milky Way galaxy is composed of ~200 billion of stars. This first version of the Kepler catalog tells us that several hundred million of them could have an exoplanet with a surface temperature adequate to sustain liquid water.

That’s a lot of interesting worlds out there, now we just need to explore them.

Comments

Thanks for the article. Very interesting findings.
Just one question: is there anything novel about the Kepler Mission in regards to the technology used, or basically we had the technology before, but not enough interest in this type of research? Could another telescope like the Hubble, for instance, have accomplished the task if it had been devoted for that task years ago?

I would say the Kepler-11 discovery bodes ill for SETI. It is worth noting that the low mass planets we are detecting in transit searches are mostly Neptune-like, the exceptions being the two that are sufficiently hot that they could be evaporated cores (CoRoT-7b, Kepler-10b). In particular, note Kepler-11f with 2.6 Earth masses but a density of 0.7 g/cc and also the case of GJ 1214b.

Where are the aliens? It may well be that the potentially-habitable worlds are covered in thick blankets of hydrogen/helium which drive the surface temperatures well above the critical point of water, or else worlds covered in utterly nutrient-poor oceans isolated from the minerals of their rocky cores by thousands of kilometres of high-pressure ices.

The first era of exoplanet detection made us wonder why our gas giants ended up in near-circular orbits out at 5 AU, rather than on elliptical paths or 3-day orbits. The Kepler era may well make us wonder why our smaller planets are built of rock and metal, rather than ice and hydrogen.

What Stanislaw Lem called “silentium universii” has always boded ill for SETI, but the outlook for life (of the lichen variety) is very good.
There is an observational bias against small planets so I never did expect true Earth analogues to crop up this soon. A couple of years ago Nature had a diagram of simulated planetary systems with various densities of the initial circumstellar disc, and various ratios of metal content. Not surprisingly, only a very few of those simulations looked like our system. But if life can evolve on those very different worlds, they will of course be adapted to deep oceans, aquifers or whatever the biosphere looks like.

Andy’s right. This data set suggests that our solar system is unusual. The most common planet type seems to be one intermediate in size between Earth and Neptune. We don’t have any of these in our solar system. Second, the few planets whose masses can be guessed at, Kepler-11, appear to be miniature gas or ice planets rather than rocky planets. Instead of ice or gas giants, we call these planets ice and gas standards (kind of like toy, miniature, and standard poodles).

Maybe ice and gas standard planets rather than terrestrial planets make up the bulk of Earth-sized planets in the galaxy, in or out of the habitable zones.

Umm…about the Kepler 11 system boding ill for extraterrestrial life…didn’t anyone see the interview with Lissauer saying that this system type is probably exceedingly rare (far less than 1%)? We knew all along that our solar system was rare, because the process of planetary formation is so stochastic (random). It probably is that only one percent of one percent of stars has a planet with complex life like ours. The rest of the systems will be interesting!

@Luiz. Kepler telescope ois designed to stare almost continuously at the same area of the skyto perform accurate photometric measurements. The Hubble Space Telescope is a multi-project telescope which is able to observe a wide variety of targets with several instruments.
The design of these facilities and their goals are quite different. The budget as well. Keple costs ~$500M, HST is a high budget project with a total cost of $5-10B.

@Andy and @Biger
You need to consider that this result is coming from only 4 months of observation with Kepler telescope. A planet at 1AU from its host star is unlikely to be detected with such small temporal baseline. Kepler is now in operation since March 2009, so in one year we will have more candidates, smaller and more likely located at larger distance from their host star. Just be patient.
This is the result of a tiny part of the Kepler Observation Program.

@Franck Marchis:
The issue is not that these planets aren’t in the habitable zone. I am fully aware of why we haven’t got any of those confirmed. The issue is that as soon as you get away from the extremely close-in (less than 1 day) orbits of planets like Kepler-10b and CoRoT-7b the super-Earths become mini-Neptunes (e.g. GJ 1214b and the planets of Kepler-11). Kepler-11f shows that this applies even for planets as small as 2 Earth masses in very close-in orbits.

This is fully consistent with the majority of low-mass planets being Neptune-like and the rocky exoplanets being evaporated cores. If anything, this implies the habitable zone planets (e.g. the worlds orbiting Gliese 581) will be even more ice/hydrogen rich as they would experience less evaporation than the relatively hot super-Earths we are already finding.

Considering how many moons orbit the planets of our solar system, I’m equally excited by the prospect of terrestrial or Europa-like (or other varieties) of moons orbiting the larger planets in or near their habitable zones.

What would the day-night cycle be like for, say, a moon orbiting a giant planet, or even an “ice standard” or “gas standard”, around the HZ? How would (potential) life adapt? What would the tides be like?

“the Kepler team extrapolated that 6% of the stars in our Milky way have Earth- and super-Earth size exoplanets, 17% of them have Neptune-size candidates and only 4% of them have Jupiter-size exoplanets.”

I could be wrong but after looking at the paper I think these results may only apply to planets with less than 100 day orbits, so if systems with planets in such tight orbits are the exception there could be far more planets in the galaxy than these figures suggest.

“Several exoplanets orbit around M- and F- type stars, which have lower temperatures than our Sun.”

Do you mean K-type instead of F-type?

Kurt-9: the information suggests our solar system is somewhat unusual, but not dramatically unusual. Maybe it really is dramatically unusual, but we don’t know that yet. They found 5 earth size planets in the habitable zone of this restricted data set. Being far enough away from the star to be in the habitable zone makes it seem unlikely that they’re evaporated cores.

Is there a theory about why multi systems seem rare ? : as a complete layman i would have thought what happened in the formation of our solar system would have been repeated everywhere ..if the basic material (and the laws of physics) are the same.

I’ve done some maths, and by my reckoning if Kepler was studying our solar system these are the odds of it detecting a single transit of each of the innermost 5 planets, first column is for 43 days of operation, second column for the time necessary for an entire orbit of the target planet:

Considering how many moons orbit the planets of our solar system, I’m equally excited by the prospect of terrestrial or Europa-like (or other varieties) of moons orbiting the larger planets in or near their habitable zones.

It’s taken most of a century to get from the first observations of distant galaxies to the presently accepted theories of the formation of the universe. SETI only goes back about half a century. The first definitive exoplanet observations occurred about fifteen years ago, and only in the last couple of years have we been getting close to identifying candidates for Earth-like planets.

Meanwhile we hardly have the last word on possible methods of communication between inhabited worlds, but there’s plenty of decent speculation to the effect that it wouldn’t be detectable by us (for example modulated lasers).

Ultimately there is no substitute for robotic missions to other star systems, that will take thousands of years (using new technologies within the scope of existing physics) but will bequeath a treasure of knowledge to our distant descendants. And there’s no substitute for the kind of long-range thinking that will lead from our present short steps into space, to the long marathons to other star systems.

You know it would be very nice if the Kepler team used actual studies of where the habitable zone is expected to be, rather than just chucking out a bunch of blackbody temperatures and saying “hey, these seem like nice values”…

Going by the habitable zone limits defined in Underwood, Jones and Sleep (2003) I have created a spreadsheet to calculate which of the Kepler candidates fall within these limits. Taking the widest definition of the habitable zone given there (recent Venus to early Mars), only 24 candidates fall into the HZ. The most conservative definition admits only 8.

The majority of the Kepler “habitable” candidates are too close to the star to be habitable and in fact have incident flux exceeding that received by Venus. It is the Gliese 581c fiasco all over again.

“The lack of more distant exoplanets in the survey is probably an observational bias since Kepler was in operation for only 4 months, so the longest orbital period attainable currently with the Kepler data is ~140 day, corresponding to a distance of 0.5 AU from an host star.”

I would have thought this, too, on the max. orbital period, but Table 6 in Borucki et al. gives orbital periods of up to 372 days for the exoplanet candidates. There’s obviously some modeling of single transits and comparison to the data going on to get P > 136 days for any candidate.

he conducts limnological and paleolimnological investigations of remote lakes and ponds in the Canadian High Arctic to characterize Holocene climate change. Darlene has also extrapolated her Arctic work to Mars analog paleolake reconstructions. Her research interests at Pavilion Lake include its chemical and biological limnological characterization, and the isotopic biosignatures associated with the microbialites.

@Andrew: this is correct, The Kepler team based these numbers on their current survey. They do not speculate on the unknown, the exoplanets orbiting far away from their host star.

@Brian: corrected thanks for pointing out this mistake

@Mark: Not yet, there are some ideas but it is too early to speculate. It may be possible that the star must form in a cocoon with high metallicity and massive enough to create enough multiple systems. Kepler survey should be able to provide clues about the multiplicity rate and the initial conditions of formation of the star. But we have to wait for the data.

@Andy: thanks for the references about the HZ. I will check it.
@Andy: I don’t have a table 6 in my draft version of the paper. The List of Planetary Candidates and their characteristics are in Table 2. The average period of all these candidates is ~35 days with 29 of them with a period larger than 140. days
Here they are
KOI period
87.0100 289.861
139.010 224.794
211.010 372.108
351.010 331.646
351.020 210.453
353.010 152.101
374.010 172.673
401.020 160.011
415.010 166.788
433.020 328.240
536.010 162.336
622.010 155.047
633.010 161.468
683.010 278.123
771.010 10389.0
806.010 143.181
881.020 226.892
998.010 161.788
1032.01 615.300
1099.01 161.525
1162.01 158.695
1230.01 165.754
1375.01 321.216
1426.03 150.034
1429.01 205.932
1486.01 254.560
1503.01 150.242
1527.01 192.674
1582.01 186.383

I am not sure where these longer periods come from. The team may have already included part of the future data?

To All: thanks for your comments, I will check this post as often as possible to reply to you. This is an interesting topic of discussion.

I would like to clarify with you that I am NOT involved in the Kepler Mission. I am a Planetary Scientist. My main interests are the study of asteroids and moons of Giant Planets. So I am writing on this topic mostly because I consider that it is an interesting topic and an important work for the future of astronomy.

@Andy
I asked Jon Jenkins from the Kepler team to give me details about the P>140 days for these targets. He told me that they used all available data to provide the most accurate and valid ephemeris information for all candidates. For instance they had access to data through Q5 for most cases. In cases they had only identified a single transit, the period was estimated assuming a circular orbit and a central transit.

Nice post, interesting thread and a heartfelt thanks to Marchis for his efforts in replying!

@ kurt9:

I would guess (layman here) that Brian Schmidt in comment 9 describes the situation well. IIRC after the first Kepler release one of the team made a big deal in his TED talk how the solar system is _not_ unusual, since if you don’t mind lousy statistics you can match the pattern of planet sizes (4 ~ Earth, 2 ~ Neptune, et cetera) precisely to Kepler’s size distribution.

So maybe Jupiter is a rare large planet, but it is still not an extreme. We know of systems with smaller and larger orbits. We now know of systems with the roughly the same number of planets, and closer packed to boot. And so on. If the solar system has an unusual (extreme, not on the observed distribution) characteristic it is still to be uncovered.

“I would have thought this, too, on the max. orbital period, but Table 6 in Borucki et al. gives orbital periods of up to 372 days for the exoplanet candidates. There’s obviously some modeling of single transits and comparison to the data going on to get P > 136 days for any candidate.”

It seems to me the observational bias is not a bias that finds planets with a ~136 day period (or less). If a planet passes in front of its star and dims it, the orbit of that planet could be anything (with either a very short, or a very long, year).

The bias comes from the fact that, with a period greater than 136 days, there is a low probability that Kepler has had a chance to see the planet pass in front of its star even once. I.E., if aliens had their own Kepler launched 136 days ago, there’d be a ~(136 / 365) chance that the Earth happened to pass in front of the Sun from their vantage point.

Question: Does Kepler filter for stars that have a planetary disk whose plane intersects our own? What about all the stars where we look top-down onto the system, thus never seeing any dimming? (What percentage of stars have a disk that we see edge-on instead of top-down?)